There's more to do in South Korea's heavily forested Rocket Valley, just a few miles from the demilitarized zone separating the two Koreas, than fire rockets. In quieter times, people tend vegetable patches along ice-cold streams.

But on Wednesday, a U.S. artillery brigade based in the South heated things up, launching a barrage of rockets close to the border town of Cheorwon.

The live-fire drills came hours after a report by reclusive North Korea that it had miniaturized nuclear warheads to be mounted on ballistic missiles, and leader Kim Jong Un had ordered further improvements to its arsenal.

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Tension in the region was already high as South Korean and U.S. troops began large-scale military exercises on Monday in a test of their defenses against North Korea, which called the drills "nuclear war moves" and threatened to respond with an all-out offensive.

The U.N. Security Council imposed harsh new sanctions on North Korea last week for its Jan. 6 nuclear test. The North launched a long-range rocket a month later, drawing international criticism and sanctions from South Korea.

The drills in Rocket Valley were separate to the annual joint U.S.-South Korean maneuvers which involve about 17,000 U.S. troops and more than 300,000 South Koreans.

One unit was dug in at the foot of Rocket Valley, under the swaying firs. A sonic boom followed the rockets as they screamed over the tree line followed by trails of flame toward targets five miles away, invisible over the ridge lines.

"If North Korea decides to use their long-range artillery, which they have so many pieces of, Seoul would be in direct range," Captain Harry Lu of the U.S. Army's 37th Field Artillery Regiment said.

"So our mission here is to make sure we destroy that artillery before they can cause any more damage to the greater Seoul metropolitan area."

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In bellicose rhetoric, North Korea routinely threatens to turn Seoul into a "sea of flames," and the city was reduced to rubble in the 1950-53 Korean conflict, which ended in a truce, not a treaty, meaning the two sides are technically still at war.

Kim Jong Un's announcement of advances in North Korea's nuclear program followed his order last week for the country to be prepared to mount pre-emptive attacks against the United States and South Korea and stand ready to use nuclear weapons.

He issued the command as the North showcased its own MLRS which is carried by a Chinese-made truck and may be able to operate outside the range of similar U.S. and South Korean weapons, according to an expert.

South Korea's defense ministry said the North's rockets flew up to 90 miles off the east coast and into the sea, a display of power seen as a response to the U.N. sanctions.

The U.S. 210th Field Artillery Brigade, based in Dongducheon, north of Seoul, is one of the only U.S. battalions that will not move to a newly expanded military base south of the capital under an agreement between South Korean and U.S. defense chiefs.

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That is because it is considered part of South Korea's "counter-fire plan" and contains MLRS, capable of firing a barrage of rockets at a target beyond the range of conventional artillery.

It is one of South Korea's first lines of defense in the event of war.

"Unless using guided munitions, [multiple-launch rockets] are less accurate than tube artillery but can put a lot of steel downrange with devastating effect," said Bruce Klinger, a senior research fellow at the Heritage Foundation and former Korea specialist at the CIA.